The present invention relates to a radio communication system, and more specifically to a radio communication system for the purpose of performing super highspeed downlink (abbreviated SDL, this abbreviation used hereinafter), which has a relatively high transmission rate on the downlink, which is the radio transmission path from the base station to the terminal, in comparison with the transmission rate on the uplink, which is the radio transmission path from the terminal to the base station.
In recent years, with advances in communications and information processing technologies, a variety of types of radio communication systems such as a personal-use portable telephone (Personal Handyphone System (PHS)) and systems which make use of the above-noted SDL have been proposed.
Radio communication systems such as PHS or local area networks (LANs) which use radio are experiencing increasing demand, by virtue of advancements in various information media, and along with this increased demand comes an increasing necessity to perform radio communication over a variety of networks. In view of this type of necessity, a broadening of the transmission frequency bandwidth is desired in radio communication systems as it is in cable communication systems. In radio communication systems of the past, the uplink for sending a radio signal from the terminal to the base station and the downlink for sending a radio signal from the base station to the terminal had transmission speeds which were matched for bi-directional radio communication. However, the actual situation is that the amount of transmission on the downlink, over which user information requested by the user is sent to the terminal is considerably larger than the amount of transmission on the uplink over which only control information or the like is sent.
This is a problem not only with mobile communication, but also with radio LANs and a variety of other radio services. However, because the frequency spectrum resources for radio are limited, it is difficult to widen the frequency bands for currently implemented services, making it desirable that higher, unused frequencies (such as sub-millimeter and millimeter bands) be developed.
FIG. 1 shows an example of the frequency placement in the past. This is the example of the Japanese digital system mobile telephone system RCR STD-27B (Research Center of Radio System STanDard 27B). In this system, the downlink and the uplink have the same transmission rate, there being systems that operate in the 800-MHz band and the 1.5-GHz band. In either of these frequency bands, the uplink and the downlink are in the same frequency band. In the 800-MHz band, the downlink is located in the range 810 MHz to 826 MHz, and the uplink is located in the range 940 MHz to 956 MHz. In previous systems, because an uplink and a downlink operating at the same transmission rate were assumed, transmission is performed in the same frequency band. However, problems arise with application to the SDL system.
In the SDL system, because a wideband downlink is assumed, in a low frequency band such as the 800-MHz band, it becomes difficult to assign this wide band and to achieve effective frequency usage. For example, in the case of trying to perform transmission at 100 MHz or so, it is obvious that it is impossible for the bandwidth to be found to allow one user 100 MHz bandwidth in the 800-MHz band. For this reason, it is necessary to perform transmission in the sub-millimeter band of several gigahertz, or in the millimeter band of several tens of gigahertz.
The US mobile telephone system can be cited as an example of a radio communication system of the past which had differing transmission rates. In this system, at the point at which a switch was being made from analog to digital, the mobile telephone handset was made to include both an analog and a digital mobile telephone, thereby enabling calling and receiving of calls in both areas. In this system, two completely different communication systems—analog and digital—are used, the handsets having few circuits in common, so that there were one each of the analog mobile telephone and the digital mobile telephone. For this reason, a problem existed in that the circuit was of a large scale.
Next, the previous method of synchronizing the signal source used as a reference will be described, using FIG. 2. FIG. 2 shows the configuration of a phase-locked loop (PLL) for the purpose of obtaining a frequency that is n/m times that of the oscillation frequency of a reference oscillator. A signal having the oscillation frequency x is frequency divided to 1/m by a frequency divider 201, and input to a phase comparator. A signal from a voltagecontrolled oscillator (the oscillator frequency of which, y, is controlled by a voltage) is frequency divided to 1/n by a frequency divider 202, and input to the phase comparator. At the phase comparator, a voltage value is output which is responsive to the phase difference of these two signals. The phase comparator output is input to a loop filter which establishes the frequency tracking characteristics of the PLL. The loop filter output is input to the voltage-controlled oscillator. The PLL is controlled so that the phase difference between the two signals at the input of the phase comparator is zero. Therefore, the following equation (1) obtains.x/m=y/n  (1)
Therefore, the output y of the voltage-controlled oscillator is as follows.y=xn/m  (2)
From the above, by means of the frequency divider 201 and the frequency divider 202, the frequency becomes n/m times and is synchronized to the reference oscillator.
By using a PLL in this manner, it is possible to obtain a signal of n/m times the frequency of and synchronized to the reference signal source. However, the method of using a PLL requires a VCO (voltage-controlled oscillator), thereby requiring a separate oscillator.
While control data and user data has been transmitted on the same radio frequency, the amount of user data was much greater than the amount of control data. In addition, user data and control data are transmitted and received separately.
It was inefficient and uneconomical to send a small amount of data over a wide transmission path. And large amounts of data require a wide transmission path.
Unless a high transmission frequency is used, it is not possible to establish a wide transmission path, and if a small amount of data is sent over a wide transmission path, it is difficult to form a transmission path, because of jitter and the like, which is caused by frequency.
As described above, to handle the transmission of diverse and large amounts of information such as in the PHS and LANs, if the transmission speeds of the uplink from the terminal to the base station and the downlink from the base station to the terminal are the same, it was not possible to make effective use of the radio circuit.
In a millimeter band such as the 60-GHz band, because of the high frequency, the electromagnetic propagation loss becomes extremely high. For this reason, when performing communication over somewhat of a distance, the transmitting power must be made large. The portable terminal of the type used in the SDL system is used in the proximity of the human body, making it unsafe from a health standpoint to transmit with high power from the terminal. A portable terminal usually is powered by a battery, and transmission with a high power leads to the problem of a shortening of the period of use before recharging or replacement of the battery.
In addition, millimeter-band devices are extremely expensive, and the requirement to use millimeter-band transmitting devices in a terminal makes it difficult to meet requirements for reduction in price. From the standpoint of volume as well, the use of millimeter-band transmitting devices makes it difficult to reduce size.
The ideal method of modulation will differ, depending upon what items of the transmitting bandwidth (transmission rate), the frequency band, the size of the transmitting/receiving circuit, the devices selected, and frequency utilization efficiency is to be given priority. For example, in narrow band communication such as in a mobile telephone, if frequency utilization efficiency is to be given priority, π/4DQPSK or QAM is used. However, in the case of wideband radio communication, these types of linear modulation require radio components that operate linearly over a wide bandwidth, making reduction of size and reduction of power consumption difficult.
In the SDL system, in which the uplink and downlink clearly have different transmission rates, if the same modulation method or methods which are similar in characteristics are used, there is no choice but to either adjust to one of the modulation methods or to use a compromise method for both the uplink and the downlink, even if performance drops.
In addition, in radio communication systems of the past, the transmission of a variety of quality information was made possible by providing radio communication systems having different transmission methods. That is, by housing two transceivers for different transmission methods in the same case, it is possible to implement diversified transmission quality. For this reason, there was the problem of the increase in size of the constitution of the transceiver.